(1) Field of the Invention
The invention relates to Electro-Discharge Machining (EDM) and more specifically to a template for dressing a new or worn EDM electrode to a predetermined length and angle.
(2) Description of the Related Art
Articles such as fuel injectors and high-temperature gas turbine engine components often contain small diameter, close tolerance holes that are drilled during their manufacture. Turbine engine components such as blades, vanes and outer shroud segments require dense patterns of small diameter holes for cooling purposes. Interior conduits introduce pressurized compressor air inside the components and the holes discharge the air as a protective film that shields the components from high temperature combustion gases. The film holes are typically 0.012 inch-0.035 inch, (0.305 mm-0.889 mm), in diameter or have comparable sized non-circular shapes. The holes are strategically positioned on the component's surface in order to ensure the most effective film protection.
Due to their extremely small size, accurate positioning requirements, and vast quantity, film holes are typically manufactured using Computerized Numerically Controlled (CNC) laser and/or Electro-Discharge Machining (EDM) work centers. Laser drilling is generally limited to near-round film holes where a line of sight exists from the laser to the film hole location. Film hole locations without a line of sight or film holes having shapes not attainable using laser are drilled using EDM. An example of an EDM work center is disclosed in U.S. Pat. No. 4,044,216 to Check et al, which is herein incorporated by reference.
EDM is a process of controlled electrical erosion of an electrically conductive material. Both the tool and the article are submerged in a circulating dielectric fluid such as hydrocarbon oil or deionized water for example. A direct current of low voltage and high amperage is delivered to the electrode at the rate of approximately 20000 Hz. The electrical impulses become sparks, which jump a 0.001-inch (0.02 mm) gap between the electrode, through the dielectric fluid, to the article. If the gap is too large, ionization of the dielectric fluid does not occur and machining cannot take place. If the gap is too small, the tool and workpiece may weld together. Intense heat is created in the localized area of the spark impact, causing the article material to melt. Molten material is expelled from the article and carried away by the dielectric fluid. The dielectric fluid also assists in dissipating the heat generated by the spark.
EDM electrodes are typically made from tungsten carbide, copper tungsten, silver tungsten, yellow brass, copper, chrome-plated materials, graphite and zinc alloys. Machinability, electrical conductivity and cost are considerations in electrode material selection; however, each application dictates the selection of the electrode material and shape. The cross sectional profile of the electrode is made in the shape of the film hole required. A single electrode may be used, or multiple electrodes may be ganged together to drill a series of holes in a particular pattern simultaneously. Electrodes may be solid or may be hollow to allow the dielectric fluid to flow through, directly to the spark gap location. During the EDM machining process, an electrode is subject to erosion caused by the sparking action. For multiple electrode setups, one or more electrodes may erode at a faster rate than others. As a result, it is difficult to maintain tight machining tolerances as an electrode gradually erodes.
Due to the extensive set up time and replacement cost of new electrodes, a trimming or finishing process is normally used to dress an eroded electrode so it may continue in service. Essentially, the eroded portion of the electrode is removed to expose a portion of the electrode with original dimensions. An electrode may need to be dressed before each consecutive article is machined or, in some applications, even more frequently. One technique for dressing an electrode is disclosed in U.S. Pat. No. 4,786,777 to Rupert. According to the '777 patent, a laser source is focused on the tip of a worn electrode to disassociate the worn tip from the electrode to reshape it in a desired, unworn configuration without the formation of burrs and whiskers on the tip end.
Another technique for dressing an electrode is disclosed in U.S. Pat. No. 4,850,147 to Baker. According to the '147 patent, a worn electrode is fed tangentially into a peripheral groove in a grinding element to impart a complimentary profile to the electrode.
Yet another technique for dressing an electrode is disclosed in U.S. Pat. No. 4,365,133 to Kiyoshi. According to the '133 patent, the worn tool electrode is positioned in close proximity to a belt electrode that travels between a supply reel and takeup reel. A DC current pulses between the worn tool electrode and the belt electrode. As a portion of the belt electrode erodes, it is advanced on the takeup reel to expose an uneroded surface for use in the next dressing cycle.
A particular disadvantage of the above-described techniques is the elaborate set up and specialty tooling requirements needed to implement them. This adds time and cost to the EDM operation.
Yet another technique for dressing an electrode 10 involves dressing the eroded electrode end 12 using a template 14 as shown in FIG. 1. An electrode 10 is first advanced in a holder 16, so an eroded end 12 extends beyond a dressed length (L), which is the distance from the holder 16 to the dressed end 18. A DC current 20 pulses between the eroded end 12, and the template 14. After a period of time, the eroded end 12 is shaped into a dressed end 18. The dressed end 18 is formed to a predetermined length (L) and angle (Alpha) by a dressing plane 22 and a dressing angle (Alpha) of the template 14.
Although the above-described technique has the advantage of utilizing less elaborate and specialized tooling, there are also disadvantages. Because the template 14 is typically made of a material with hardness that is identical or comparable to the electrode 10, the template 14 develops holes 24 caused by extended spark erosion. The depth (D) of the holes 24 increases with the number of electrode 10 dressings. As the depth (D) of the erosion holes 24 increases, each successive eroded end 12 is only moderately dressed. Instead of being dressed to a length (L), the dressed end 18 is dressed to a length greater than length (L). A moderately dressed end 18 is longer than required and can lead to article defects such as, an unacceptably deep hole, incorrect hole positioning, incorrect hole shape, or a short circuit between the electrode 10 and the article at the start of the EDM operation. Since none of these conditions is acceptable and can lead to damaged articles, dressing templates 14 are frequently replaced. Frequent replacement of dressing templates 14 is time consuming and adds additional cost to the EDM operation.
What is therefore needed is an EDM electrode dressing template 14 that is less susceptible to spark erosion, thereby improving hole quality and reducing the time and expense of the EDM operation.